This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/SG99/00022 which has an International filing date of Mar. 30, 1999, which designated the United States of America.
This invention relates to a portable apparatus primarily for use in the field and for measuring the drainage performance of porous asphalt road mixes, using the falling head principle. Porous road mixes are relatively new road making materials in the road industry particularly in South East Asia.
With porous (or drainage) road mixes there is no systematic way of measuring the drainage performance. The impetus for use of these types of mixes in the tropics with high and frequent rainfall is their ability to rapidly drain off rain water so as to provide a dry riding surface even in heavy downpours. This will greatly enhance road safety as a dry riding surface would eliminate the problem of wet skidding, aquaplaning, and reduced visibility caused by surface water spray from vehicle tires.
There are some existing field apparatus for comparing the drainage capability of porous asphalt road slabs. These include a Belgian permeameter (Heystraeten, 1990), a Swiss IVT permeameter (Isenring et al, 1990), and a Spanish LCS drainometer (Ruiz et al, 1990). All these apparatus have the common feature of a transparent cylindrical tube in which the time for a falling head of water between a fixed distance along the tube is measured by a stop watch. Thus, by taking the measurement of time for a fixed volume of water to flow from the cylinder into the porous road slab, the average flow rate of the water discharge through the pavement can be estimated. This flow rate is then a kind of empirical measure of the drainage capability of the porous asphalt pavement. The variation in such equipment is basically: (a) size of cylindrical tube and volume of water used for test; (b) the way sealing is achieved between the cylinder and the road slab; (c) the way water is retained above the road slab before start of test; and (d) the way time is measured between the start to end of test. Presently, all existing apparatus do not have the sophistication to allow for a rational and objective determination of the permeability or drainage property of the porous road tested.
One object of this invention is to provide a simple and inexpensive field apparatus which will enable the accurate measurement of the drainage property of porous road mixes using very simple technology and components. The device should be able to measure the permeability of porous road mixes using established fluid mechanics principles, where the permeability is a measure of the ease with which the mixes allow the flow of water through the pores.
In accordance with this invention, there is provided a field apparatus for the determination of the drainage properties of a porous asphalt road surface or slab in terms of its permeability coefficient (k) and flow exponent (m) using a falling head test method, which apparatus is characterised by:
(i) a water containing cylinder,
(ii) weighting means to provide pressure for sealing a rubber lined flanged base of the cylinder to the road surface,
(iii) a releasable means to provide a water retaining closure at the base of the cylinder to hold back water in the cylinder before the start of test, and to release the water suddenly from the cylinder onto the road surface at the start of test, and
(iv) a pressure transducer located in a lower part of the cylinder for measurement of the falling head of water in the cylinder during the test.
Features of this invention include the preferred use of a mechanical trap to retain water above the road slab before the start of test and to activate the sudden release of water at the start of test and the use of an accurate pressure transducer near the base of the tube to measure the falling head of water at small intervals, such as every millisecond. In addition, a theoretical basis is provided for reducing the falling head data to obtain a direct measure of the permeability of the porous road section tested.